Hydration behavior and immobilization mechanism of MgO-SiO₂-H₂O cementitious system blended with MSWI fly ash

MSWI(Municipal solid waste incineration) fly ash as hazardous wastes usually is treated properly with OPC(ordinary Portland cement) based binders prior to being disposed in security landfill, but some toxic elements present poor stability in its highly alkaline environment. This study developed a novel solidification/stabilization technology based on MgO-SiO₂-H₂O (M-S-H) cementious material with compatibility with contaminants for MSWI fly ash encapsulation. Compressive strength and leaching toxicity tests were undertaken to evaluate the immobilization effect of MSWI fly ash treated with M-S-H matrix. And TAM, XRD, DTA/TGA and SEM techniques were employed to explore hydration process and microstructure morphology of M-S-H solidified body. Results demonstrated that M-S-H cement matrix played a positive impact on the immobilization of heavy metals, and the immobilization rate of Cd, Pb and Zn was 97.5%, 99.8% and 98.7%, which was far more than that fixed in Portland cement. The incorporation of MSWI fly ash inhibited the hydration process of M-S-H matrix on the whole, which was mainly due to its poor pozzolanic activity. Moreover, there were some new hydration phases including CdSiO₃, Ca₃PbSiO₆ and Zn(OH)₂ generated and some MSWI fly ash embedded in the core-shell structure of M-S-H matrix in depth. These results suggested that heavy metals of MSWI fly ash were mainly fixed in M-S-H solidified bodies by physical encapsulation, isomorphous replacement and chemical precipitation. Overall, this study demonstrated that M-S-H matrix is a promising candidate that can serve as low-carbon and high-efficient materials for hazardous MSWI fly ash.